1. Field of the Invention
The present invention relates to a fuel gas production method and apparatus for reforming a fuel containing hydrocarbon or alcohol to produce a hydrogen-rich fuel gas.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between a pair of separators. The membrane electrode assembly and the separators make up a unit cell for generating electricity. Generally, a plurality of the unit cells are stacked together to form a fuel cell stack. The fuel cell stack has been used widely in various applications including stationary and mobile applications.
In the unit cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current.
Conventionally, hydrocarbon fuels such as natural gas or hydrogen-containing fuels such as alcohols (e.g. methanol) are refined to produce the hydrogen-containing gas as the fuel gas, and the hydrogen-containing gas is supplied to the fuel cell stack.
For example, Japanese laid-open patent publication No. 2000-20102 discloses a hydrogen production apparatus as shown in FIG. 8. The hydrogen production apparatus includes a compressor 1, a hydrodesulfurization unit 2, a steam reformer 3, a catalyst combustor 4, a gas shift converter 5, and a PSA (Pressure Swing Adsorption) unit 6. A fuel such as city gas is supplied from the compressor 1 to the hydrodesulfurization unit 2. After sulfur is removed from the fuel, the fuel is reformed by the steam reformer 3 to produce a gas having a high hydrogen-concentration (hydrogen-containing gas). The catalyst combustor 4 is provided around the steam reformer 3 for inducing combustion of hydrogen and oxygen in the air by the action of the catalyst. The gas shift converter 5 converts carbon monoxide in the hydrogen-containing gas into carbon dioxide and hydrogen. After the gas shift reaction, the PSA unit 6 produces highly pure hydrogen from the hydrogen-containing gas by pressure swing adsorption.
A hydrogen tank 8 and an off gas holder 9 are connected to the PSA unit 6. The hydrogen tank 8 temporarily stores pure hydrogen before it is supplied to a polymer electrolyte fuel cell (PEFC) 7. The off gas holder 9 temporarily stores the off gas (impurities) collected by pressure swing adsorption of the PSA unit 6. The off gas holder 9 supplies the off gas to the catalyst combustor 4 as a fuel for heating the steam reformer 3.
The PAS unit 6 has a plurality of adsorption towers filed with adsorbent material for selectively absorbing impurities (components other than hydrogen) under high pressure, and releasing the absorbed components under low pressure. The impurities in the hydrogen-containing gas are absorbed by the adsorption towers under high pressure leaving the hydrogen in the gas container, and the hydrogen is removed as the purified hydrogen product. After the hydrogen is removed, the impurities are released from the adsorption towers under low pressure. The waste gas containing the impurities is discharged from the PSA unit 6 as the off gas. Series of operations, i.e., absorption of the impurities, reduction of pressure to release the impurities, replacement of the gas, and pressure increase are carried out as an cycle for collecting the highly pure hydrogen, and discharging the other components as the off gas.
The off gas holder 9 has a considerably large volume for maintaining the desired pressure difference in the PSA unit 6. A large space is required for installing the off gas holder 9. The hydrogen production apparatus is considerably large as a whole.
Therefore, it is suggested to directly supply the off gas to the catalyst combustor 4 from the PSA unit 6 without using the off gas holder 9. In this case, the off gas discharged from the PSA unit 6 is affected by pressure pulsation. Thus, the amount of the off gas supplied to the PSA unit 6 to the catalyst combustor 4 fluctuates undesirably. The temperature of the catalyst of the catalyst combustor 4 is likely to fluctuate.
In particular, when the temperature of the catalyst is low, CO and unreformed HC in the off gas are not fully combusted in the catalyst combustor 4, and discharged directly to the outside. For example, at the temperature of 500° C. or less, methane (CH4) is not fully used in the combustion induced by the action of the catalyst, and the unused methane is discharged as an exhaust gas to the outside. Therefore, the exhaust gas is not clean.